In 2151, Phlox questioned Elizabeth Cutler about why she thought that the heart was the source of emotion, since it was "nothing more than a very efficient pump." She responded by saying "You know, you may know about our cardiopulmonary system, but you have a lot to learn about the Human heart." (ENT: "Dear Doctor")

The following year, when a creature attached itself to B'Elanna Torres, The Doctor could not remove the creature without causing irreparable damage to Torres' organs. Janeway suggested beaming it out, but this was deemed impossible to do without also removing Torres' cardiopulmonary system along with it. (VOY: "Nothing Human")